Make your way to the Children’s Zoo, and you will find one of the most unique mammal species in the world: the naked mole-rat Heterocephalus glaber. More closely related to the capybara and porcupine than they are to moles or rats, naked mole-rats have many attractive characteristics, beyond their conspicuous superficial beauty. For example, they live longer than any other rodent, with a life span of nearly 30 years in zoos. Despite living to this relatively old age, naked mole-rats appear to be remarkably resistant to cancer, making them a promising model for age-based disease. Perhaps most interestingly, they are unique among mammals in having a highly structured social organization called eusociality, similar to the social structure of most ants, bees, and termites. Their completely subterranean colonies consist of only one breeding “queen,” a few breeding males, and the remaining individuals, which behave as nonbreeding subordinate workers. Among the subordinate naked mole-rats, there are no observable differences between males and females. This poses a problem for zoos, because it is important to know if individuals are male or female. Fortunately, we have a tool that allows us to look beyond the physical characteristics: genetics!
The Genetics Division at the San Diego Zoo Institute for Conservation Research routinely engages in molecular sexing of birds when it is not possible to distinguish males and females. A recently posted blog titled Boy Or Girl? Genetic Testing described this technique as it’s done for the California condor. However, we rarely use these types of techniques for mammals because it’s usually unnecessary. And until recently, we never attempted genetic sexing of the naked mole-rat. Fortunately, a recently published study described a method designed to do just that.
Like most mammals (including humans), naked mole-rats have an XY sex-determination system: XY=male, XX=female. The method we followed takes advantage of this system. The basic idea is to detect the presence or absence of a Y chromosome, which would indicate a male or female, respectively. To do so, we used synthetic DNA molecules called “primers,” which are designed to match the naked mole-rat’s DNA, and utilized a process called polymerase chain reaction (PCR), which amplifies millions of copies of a gene, or part of a gene, of interest. In this case, we amplified a portion of the sex-determining region on the Y chromosome. We also co-amplified a gene found in both sexes called “16S” to make sure that the absence of a Y chromosome is consistent with a female, versus simply poor quality DNA from a male. Once the genes are amplified, they are visually analyzed in a process called gel electrophoresis.
The results were unmistakable! Using DNA collected noninvasively via cheek swabs, we were able to genetically sex every sample when compared with the known controls. Pictured below is the result of one group of naked mole-rats. As you can see, the five newly identified males have two bands, corresponding to the 16S and Y genes, while the six females have one band, corresponding to just the 16S gene.
Steven Thomas is a senior research technician at the San Diego Zoo Institute for Conservation Research.